1. Field of the Invention
The present invention relates to a radio wave source information display apparatus in which exponential function components contained in a radio wave source spectrum signal obtained in the process of applying a deconvolutional method to antenna response are extracted so as to directly obtain and display radio wave source information.
2. Description of the Related Art
When observing target objects, for example, in using a radar, a method has been generally used in which the pointing direction of an antenna beam is changed, for example, by rotating the antenna to received radio waves from the respective azimuths pointed to by the antenna beam, so as to observe the intensity of the received radio wave (antenna response) with respect to the azimuths. In doing so, the use of an antenna with a narrower beamwidth results in an antenna response approximating the distribution of the radio wave sources and, hence, improves the azimuth resolution of the radar. To improve antenna resolution in the conventional art thus means to obtain an antenna response more closely approximating the distribution of the radio wave sources. Although another method is also known such as in a synthetic aperture radar where antenna resolution is improved by subjecting the received radio wave to a signal processing, this method, too, intents to obtain an antenna response approximating the distribution of the radio wave sources by achieving through the signal processing an effect equivalent to that of reducing the antenna beamwidth.
The above conventional technique for improving the antenna resolution is a method of indirectly obtaining the distribution of the radio wave sources from the antenna response and has a problem that the distribution of the radio wave sources cannot be directly obtained. If there existed an antenna having its pattern represented by the Dirac delta function, the antenna response at such antenna would correspond to the response of the radio wave sources. It is known from the antenna theory, however, that an antenna having such a pattern does not exist. Accordingly, since an actual antenna pattern has a finite beamwidth and sidelobes, there is a problem that the distribution of the observable radio wave sources is distorted by the antenna pattern.
By contrast, a method is well known as a technique for improving resolution, for example, of radar in which a radio wave source distribution function is obtained by using a deconvolution method to improve the resolution. The technique for obtaining a radio wave source distribution function by using the deconvolution method includes the steps of: effecting a Fourier transform with respect to azimuth of a received electric field signal obtained from the antenna while moving the antenna beam; effecting a Fourier transform with respect to azimuth of a received electric field signal pattern of the antenna in the presence of one point wave source; dividing a signal resulting from the Fourier transform with respect to azimuth of said antenna received electric field signal by a signal resulting from the Fourier transform with respect to azimuth of said received electric field signal pattern of the antenna in the presence of one point wave source; and subjecting the divided signal to a Fourier inverse transform with respect to azimuth, the Fourier inverse transform signal being outputted as a final antenna output.
The operation for obtaining radio wave source distribution function by the deconvolution method is explained in further detail as follows. In particular, supposing .theta. is the azimuth, g(.theta.) is an antenna pattern and f(.theta.) is a wave source distribution function, an antenna-received electric field e(.theta.) is given by the form of a convolution integral as in [the] equation (1). EQU e(.theta.)=.intg.f(.theta.).multidot.g(.theta.-.phi.)d.phi. (1)
It should be noted that f(.theta.( in the equation (1) is identical [as] to the wave source distribution function f(.theta.) and .phi., representing an integral variable (an expedient variable in the integral equation), has the same unit of azimuth as .theta..
In general, the antenna pattern g(.theta.) is measured as an electric field received at the antenna in the presence of one point source of wave. Here, supposing E(.omega.), F(.omega.), G(.omega.) as the functions resulting from Fourier transform in respect of azimuth, respectively, of e(.theta.), f(.theta.), g(.theta.) i.e., as azimuthal frequency functions, the equation (1) may be represented by the form of a multiplication as in the following equation (2). EQU E(.omega.)=F(.omega.).multidot.G(.omega.) (2)
where G(.omega.) is an azimuthal frequency function of antenna pattern, i.e., a transfer function with respect to azimuthal frequency of the antenna. Since the antenna pattern g(.theta.) is determined when the antenna to be used is decided, G(.omega.) can be obtained by calculation from g(.theta.). Further, E(.omega.) is an azimuthal frequency function of the antenna-received electric field e(.theta.) and can be obtained by calculation from a measured value of the electric field signal e(.theta.) received by the antenna at each pointing angle. Accordingly, E(.omega.), G(.omega.) are known and the azimuthal frequency distribution function F(.omega.) of the wave source can be obtained by EQU F(.omega.)=E(.omega.)/G(.omega.) (3)
As described above, F(.omega.) is the Fourier transform with respect to azimuth of the distribution function f(.theta.) of the wave source. It is therefore possible to obtain the wave source distribution function f(.theta.) by a Fourier inverse transform with respect to azimuth of F(.omega.) which is represented by the equation (3). It should be noted that ".omega." represents spatial frequency.
Here, the above described known technique for improving antenna resolution by using the deconvolution method is to obtain only the radio wave source distribution by removing skewness of antenna pattern from antenna response which is a radio wave source distribution skewed by antenna pattern. Fundamentally in the deconvolution method, the signal resulting from Fourier transform of antenna response in respect of azimuth is divided as described above by the signal resulting from Fourier transform of antenna pattern with respect to azimuth, or after multiplying the signal resulting from Fourier transform of antenna response with respect to azimuth by an inverse filter based on the antenna pattern, such signal is subjected to Fourier inverse transform with respect to azimuth. Thereby a radio wave source distribution function is obtained.
In actuality, however, the radio wave source distribution is not obtained in the form of a function even when the deconvolution method is used, and it is given merely as function values for respective azimuths. Accordingly, a further processing is necessary with the deconvolution method to extract radio wave source information from the signal after Fourier inverse transform is performed. Usually, for example, the signal after Fourier inverse transform is graphed and processing is effected, for example, to obtain the position and size of radio wave source from the graph. The deconvolution method thus has a problem in that radio wave source information cannot be directly extracted.